As known, the automatic pilot (AP automatic piloting system or FD flight director) of a transport airplane, either coupled or not to a thrust regulation system, uses speed information in order to maintain a selection of the pilot and/or the speed field to values acceptable for the airplane.
If, during piloting, such speed information is lost, that is if it becomes unavailable, erroneous, or unreliable, the automatic pilot (AP automatic piloting system, FD flight director) and/or the thrust regulation system automatically disengage and/or maintain the current state of the airplane, leaving to the crew the charge of managing the situation.
Such a situation could occur, for example should the Pilot probes generally used for measuring the speed of the airplane, become iced-up. Such a situation is however exceptional, as most airplanes are provided with systems allowing the effects of icing to be limited or cancelled.
A disengagement of the automatic piloting system, the flight director and/or the thrust regulation system induces an additional workload for the crew that should process the cause of the breakdown, in addition to the usual tasks, and this, in an unusual situation wherein the speed information is lost.
The speed information is a primary parameter used by the automatic piloting system or the flight director, for defining the flight area of the airplane (high and low limits) and the dynamics of the airplane. An automatic pilot (AP, FD) or a thrust regulation system that would not respect such speed limits would lead the airplane to be unstalled or in an overspeed situation able to lead to structural damages. For this reason, the automatic pilot and the thrust regulation system are provided with different usual means protecting the speed area.
Consequently, when the speed information is lost, it is not possible to ensure the basic function of the automatic pilot (AP, FD) and the thrust regulation system in their current concepts, nor the protective functions of the flight area.
It should be furthermore noticed that a current automatic piloting system, a flight director and a thrust regulation system contain a high number of different operating modes each corresponding to an objective the crew can select as a function of their intentions. Learning such different modes and the transitions thereof represents a significant part of the crews' training. Thus, introducing new operating modes of the automatic piloting system, of the flight director and/or of the thrust regulation system generally requires a new learning phase, even of repeated training phase, if this mode is only of use in exceptional cases that generally do not occur in a normal use of the airplane.